This invention relates to a photoemitter capable of operating as a cathode in an electron tube.
No material has yet been found that has such a small energy gap and such a low work function as are practically suitable for a photoemitter having sensitivity in the long-wavelength range (longer than 1 .mu.m). To obtain the sensitivity in the long-wavelength range, there have been proposed some prior art devices using the photoemitters which have their energy-band diagrams as shown in FIGS. 1-3 and corresponding structures as shown in FIGS. 1A, 2A, and 3A.
FIG. 1 is an energy-band diagram of a NEA (negative electron affinity)-type photoemitter fabricated by applying Cs-O treatment to a semiconductor. FIG. 1A shows the corresponding structure of the photoemitter having the energy-band diagram shown in FIG. 1. In FIGS. 1 and 1A, a p-type GaAs semiconductor substrate is shown by numeral 11 which is mounted on metal base 13, and a Cs-O compound layer joined to the substrate surface by adsorption is indicated by 12. In FIG. 1, symbols E.sub.c, E.sub.f, E.sub.v and E.sub.o denote the energy level at the top of the conduction band, the Fermi level, the energy level at the bottom of the valence band, and the vacuum level, respectively. The structure shown in FIG. 1A achieves reduction in work function by joining the surface level layer to the Cs-O compound layer 12.
FIG. 2 is an energy-band diagram of a photoemitter in which a p-n junction is formed in a Ge semiconductor and further the Cs-O treatment (not shown) is applied. FIG. 2A shows the corresponding structure of the photoemitter having the energy-band diagram shown in FIG. 2. As shown in FIG. 2A, a p-n junction is formed between a p-type Ge semiconductor 21 and an n-type Ge semiconductor 22. An electrode (not shown) is formed on the p-type Ge semiconductor 21 at the side opposite to the p-n junction. A partial electrode (also not shown) whose area is small enough to avoid affecting the photoemission or light incidence is formed on the n-type Ge semiconductor 22 at the side opposite to the p-n junction, i.e., at the side of a photoemitting surface. The surface barrier height of the n-type Ge semiconductor 22 is reduced by adsorption of Cs-O layer 24 on the photoemitting surface of the n-type Ge semiconductor 22. A depletion layer 23 is formed by the p-n junction and a bias voltage. The photoemitter structure is mounted on metal base 25. The structure shown in FIG. 2 achieves a substantial reduction in work function by the combined effect of the p-n junction and the reverse bias. It is noted that similar results can be attained by using a Schottky junction instead of the p-n junction.
FIG. 3 is an energy-band diagram of the photoemitter shown in FIG. 3A wherein a junction is formed between a p-type InGaAs semiconductor 31 (a material having a small energy gap) and an InP semiconductor 32 (a material having a large energy gap) and further a Cs-O layer 34 is applied to the photoemitting surface of the n-type InP semiconductor 31. An electrode (not shown) is formed on the semiconductor 31 at the side opposite of the junction, and a partial electrode (also not shown) whose area is small enough to avoid affecting photoemission or light incidence is formed on the semiconductor 32 at the side opposite to the junction, i.e., at the side of the photoemitting surface. The surface barrier height of the semiconductor 32 is reduced by adsorption of the Cs-O layer 34. A depletion layer 33 is formed by the semiconductor junction and a bias voltage. The photoemitter structure is mounted on metal base 25. The heart of the structure shown in FIG. 3A is that a material having a small energy gap and a material having a large energy gap are processed to form a junction with care being taken to minimize the interfacial barrier height in the conduction band. Further, The surface barrier height is reduced by application of a bias voltage or by some other means. Thus, a photoemitter having sensitivity in the long-wavelength range can be fabricated.
These conventional types of photoemitters which are either in the laboratory stage or commercialized are characterized in that photoelectrons are created by inter-band transition in a semiconductor and that those photoelectrons are transferred into a material having a low electron affinity by various methods and then are emitted outside.
As is understood from the foregoing description, the long-wavelength limit for the emission of photoelectrons from conventional photoemitters cannot be made longer than the wavelength determined by the energy gap of a semiconductor. In the presence of a surface barrier at the emitting surface, the long-wavelength limit is further shortened by its barrier height. Hence, in order to make a photoemitter having sensitivity in the long-wavelength range, not only is it necessary to use a semiconductor having a small energy gap but also the substantial surface barrier height must be reduced by one of the methods described above.
However, in order to achieve the reduction in the substantial surface barrier height by using a Cs-O layer as shown in FIG. 1A, the semiconductor used must have an ultraclean surface. In addition, such a clean semiconductor must form a junction with the Cs-O layer without creating an energy barrier in the conduction band. These requirements can only be met by a very sophisticated technique, and the semiconductors that can be used are also very limited.
In order to fabricate a photoemitter of the type shown in FIG. 2A, the p-n junction should have a very high breakdown voltage, because in order for photoelectrons to be emitted from the semiconductor surface while retaining the energy acquired at the p-n junction, the total thickness of the n-type layer and the depletion layer must not exceed the mean free path of the photoelectrons. Further, a reverse bias voltage high enough to overcome the surface barrier must be applied to the thin depletion layer, creating an extremely strong electric field there. This will typically cause Zener breakdown, thus making application of the reverse bias voltage impossible. What is more, semiconductors having the smaller energy gap, in general, are more likely to fail by Zener breakdown and this has been one of the biggest obstacles to the previous attempts to fabricate a desired photoemitter (i.e., having sensitivity in the long-wavelength range) by the approach shown in FIG. 2A. Even if Zener breakdown does not occur, the increase in the reverse saturation current will straightforwardly result in an increased dark current, and this causes a problem in the semiconductor material having a small energy gap. Thus, it has been difficult and impracticable to fabricate photoemitters of the type shown in FIG. 2A.
In fabricating a photoemitter of the type shown in FIG. 3A, it is important that a junction be formed without creating a barrier in the conduction band. In the presence of such a barrier, photoelectrons must have an energy beyond the barrier height and the long-wavelength limit is accordingly shortened. This barrier normally becomes high and few combinations of semiconductors are known that are capable of extending the wavelength limit into the infrared range. Further, in general, recombination centers are likely to be created at the interface of a semiconductor heterojunction and it is impossible to transfer photoelectrons with high efficiency. Hence, most of the photoemitters of the type under consideration that have been realized successfully are limited to the combinations of semiconductor materials having very similar properties. In some cases, a junction is formed between semiconductors having fairly different properties as shown in FIG. 3A but they provide only low sensitivity. In many other cases, a junction is formed between a III-V semiconductor and a ternary or quaternary semiconductor of the same families, but this approach still involves many problems such as a limited ratio of a mixed crystal and the need for adopting a very sophisticated technique.
These problems are chiefly due to the fact that the two requirements must be met at the same time; one for using a semiconductor of a small energy gap to achieve efficient photoemission by inter-band transition in a semiconductor, and the other for reducing the surface barrier height.
A photoconductor is also known that generates photoelectrons not by the inter-band transition in a semiconductor but in the barrier created by a semiconductor-metal Schottky junction. By generating photoelectrons or holes by internal photoemission from the Schottky barrier, this detector has sensitivity in the long-wavelength range. However, this detector is classified as a photodiode and no case has been known in which the photoelectrons generated in the Schottky junction are emitted outward.